1. Field of the Invention
This invention relates to a process for making perchloroethylene by thermal, noncatalytic pyrolysis of carbon tetrachloride in the presence of elemental hydrogen and chlorine at relatively low temperatures. In particular, it relates to a process for production of perchloroethylene and hydrogen chloride using carbon tetrachloride as both the principal reactant and a reactive diluent under conditions which maximize consumption of unwanted carbon tetrachloride, while minimizing the production of heavy ends, such as hexachlorobenzene and other tarry products.
2. Background of the Invention
When conventionally manufacturing perchloroethylene by chlorination of hydrocarbons and/or their partially chlorinated derivatives, substantial amounts of carbon tetrachloride are also obtained. In addition, substantial quantities of undesirable highly chlorinated products such as hexachloroethane, hexachlorobutadiene and hexachlorobenzene, hereinafter referred to as heavy ends, are formed. Carbon tetrachloride, however, is thought to be among the halocarbons which cause destruction of the ozone layer and is therefore coming to be considered a relatively undesirable product. It has also been used as a feedstock in producing environmentally deleterious fully halogenated chlorofluorocarbons and demand for it is therefore decreasing for this reason as well. Because of these undesirable environmental attributes of carbon tetrachloride, regulations governing the production and use of carbon tetrachloride are expected to result in a marked decrease in demand for carbon tetrachloride over the next decade. The heavy ends resulting from conventional processes for producing perchloroethylene are also undesirable and their disposal by burning can result in emission of undesirable compounds into the atmosphere. On the other hand, the ecologically more benign chlorinated hydrocarbons, notably perchloroethylene, are expected to remain in high demand because of their many practical uses, both as a solvent and as a starting material for the production of other chemicals. The present invention addresses this situation by providing a process for the production of perchloroethylene that consumes carbon tetrachloride and minimizes formation of heavy ends.
Perchloroethylene can be produced by pyrolysis of carbon tetrachloride at high temperatures as mentioned in U.S. Pat. No. 1,930,350. As explained in U.S. Pat. No. 3,364,272, the pyrolysis process for production of perchloroethylene ordinarily requires reaction temperatures of the order of 800.degree. C. The pyrolysis of carbon tetrachloride discussed in U.S. Pat. No. 2,447,410, requires a temperature of 1300.degree. to 1400.degree. C. The production of perchloroethylene at these high temperatures, however, has serious disadvantages. Notably, the use of the high pyrolysis temperatures generally requires high energy input to initiate and maintain the reaction, expensive materials for reactor construction, and elaborate product separation to remove the unwanted heavy ends.
Catalytic systems have also been used to produce perchloroethylene. U.S. Pat. No. 4,002,695, for instance, discloses a process for preparing perchloroethylene by reaction of carbon tetrachloride vapor with hydrogen in the presence of a barium chloride catalyst at a temperature of at least 500.degree. C. However, the catalysts required in this method are expensive and subject to deactivation due to fouling with carbon. The production of unwanted heavy ends, such as hexachloroethane, hexachlorobutadiene and hexachlorobenzene, is also promoted by the use of such catalyst.